Insertion compounds are those that act as a host solid for the reversible insertion of guest atoms. Such compounds, particularly lithium insertion compounds, have been extensively studied in recent years for purposes that include use as electrode materials in advanced high energy density batteries. A new type of rechargeable lithium battery, known as a lithium-ion battery, is a product based on lithium insertion compounds that has just become available commercially. These batteries have the greatest energy density (Wh/L) of presently available conventional rechargeable systems (ie. NiCd, NiMH, or lead acid batteries) and represent a preferred rechargeable power source for many consumer electronics applications. Additionally, lithium ion batteries operate around 31/2 volts which is often sufficiently high such that a single cell can suffice for many electronics applications.
Lithium ion batteries use two different insertion compounds for the active cathode and anode materials. The excellent reversibility of lithium insertion makes such compounds function extremely well in rechargeable battery applications wherein thousands of battery cycles can be obtained. In a lithium ion battery, lithium is extracted from the anode material while lithium is concurrently inserted into the cathode on discharge of the battery. The reverse processes occur on recharge of the battery. Lithium atoms travel or "rock" from one electrode to the other as ions dissolved in a non-aqueous electrolyte with the associated electrons travelling in the circuit external to the battery.
The first commercial lithium ion batteries were 3.6 volt products based on LiCoO.sub.2 /pre-graphitic carbon electrochemistry from Sony Energy Tec. However, a wide range of carbonaceous compounds is suitable for use as the anode material, including coke (described in U.S. Pat. No. 4,702,977) and pure graphite (described in U.S. Pat. No. 4,423,125). Additionally, many other lithium transition metal oxide compounds are suitable for use as the cathode material, including LiNiO.sub.2 (described in U.S. Pat. No. 4,302,518) and LiMn.sub.2 O.sub.4 (described in U.S. Pat. No. 4,507,371). In the preceding, non-aqueous electrolytes are employed comprising LiBF.sub.4 or LiPF.sub.6 salts and solvent mixtures of ethylene carbonate, propylene carbonate, diethyl carbonate, and the like. Again, numerous options for the choice of salts and/or solvents in such batteries are known to exist in the art.
LiMn.sub.2 O.sub.4 is a particularly attractive cathode material candidate because manganese is significantly cheaper than cobalt and/or nickel. LiMn.sub.2 O.sub.4 refers to a stoichiometric lithium manganese oxide with a spinel crystal structure. This stoichiometric compound however has been found to exhibit undesirably poor cycle life when used as the cathode in lithium ion batteries of conventional construction. These cycle life problems have been overcome by varying the stoichiometry and methods of synthesis to some extent.
M. M. Thackeray et al. in U.S. Pat. No. 5,316,877 show a capacity versus cycle life improvement that results from incorporating additional lithium into the conventional stoichiometric spinel LiMn.sub.2 O.sub.4. Therein, the compound was denoted as Li.sub.1 D.sub.x/b Mn.sub.2-x O.sub.4+.delta.. When D is Li, .delta. is 0 and the improved compound can be denoted Li.sub.1/x Mn.sub.2-x O.sub.4. wherein 0.ltoreq.x&lt;0.33. Although it was suggested that other metals D could be incorporated with similar beneficial results, no mention was made therein regarding electrochemical behaviour of these compounds above 4.5 volts versus Li/Li.sup.+. Actual electrochemical testing was restricted to voltages below 4.5 V versus Li/Li.sup.+.
At the 11th International Seminar on Primary and Secondary Battery Technology and Application, Feb. 28-Mar. 3, 1994, Deerfield Beach, Fla., J. M. Tarascon confirmed the preceding result regarding capacity versus cycle life improvement by incorporating additional lithium into LiMn.sub.2 O.sub.4. In this presentation, cyclic voltammograms also showed that both LiMn.sub.2 O.sub.4 and compounds comprising additional lithium exhibit small capacity peaks at around 4.5 volts and 4.9 volts versus Li/Li.sup.+. The small 4.5 V capacity decreased as the amount of additional lithium incorporated increased. On the other hand, the small 4.9 V capacity increased as the amount of additional lithium incorporated increased. It was noted that while some capacity was observed at voltages above 4.5 V and that this capacity increased with additional lithium content, this capacity was nonetheless extremely small. The useful capacity of the compound was obtained at voltages below 4.5 V and battery testing was confined to voltages below 4.5 V. In this presentation, no mention was made about nickel or chromium substituted compounds nor about the possibility of obtaining significant capacity at voltages above 4.5 V in such compounds.
In another article by the same author, J. M. Tarascon et al., J. Electrochem. Soc. Vol. 138, No. 10, (1991) 2859, the electrochemical properties of LiMn.sub.2 O.sub.4 and various substituted compounds thereof were investigated. Included in the investigation were compounds Li.sub.x Mn.sub.2-y M.sub.y O.sub.4 (which can also be denoted as Li.sub.x Ni.sub.z Mn.sub.2-z O.sub.4) wherein y ranged up to values of 0.4. However, no electrochemical testing was performed above 4.5 volts versus Li/Li.sup.+. It was indicated therein that the lithium capacity of these nickel containing compounds decreases with increasing nickel content. Thus these nickel containing compounds appeared unattractive for battery applications.
Many substituted LiMn.sub.2 O.sub.4 spinel insertion compounds have been reported in the art for application in lithium batteries. Much mention has been made of nickel and/or chromium substituted LiMn.sub.2 O.sub.4 spinel insertion compounds. For example, in laid-open Japanese Patent Application No. 04-141954, Matsushita report on Ni and Cr substituted compounds wherein up to 0.2 moles of transition metal was substituted per molecule of LiMn.sub.2 O.sub.4. Again, however, electrochemical testing was restricted to voltages below 4.5 V. In the Journal of Power Sources, 43-44 (1993) 539-546, W. Baochen et al. show that up to one chromium atom can be substituted for manganese in one mole of LiMn.sub.2 O.sub.4. Again, electrochemical testing was performed at voltages only up to 3.8 V versus Li/Li.sup.+. In U.S. Pat. No. 5,084,366, Y. Toyoguchi demonstrated that a cycling benefit could be obtained by appropriate chromium substitution. Again, electrochemical testing was performed only up to 4.5 V versus Li/Li.sup.+.
Historically, difficulties with electrolyte stability has hindered the electrochemical evaluation of insertion compounds at relatively high voltages with respect to lithium. However, progress has been made in this area. U.S. Pat. No. 5,192,629, for instance, reveals a non-aqueous electrolyte suitable for use in a lithium ion battery comprising a Li.sub.1+x Mn.sub.2 O.sub.4 cathode that is oxidation resistant up to about 5 V.
With the availability of electrolytes that are stable at high voltage, lithium insertion compounds that deliver useable capacity at voltages above 4.5 V versus Li/Li.sup.+ can not only be studied more easily but can also be considered for use in commercial battery applications. For instance, it was shown in Canadian Patent Application Serial No. 2,102,738 that a related group of lithium insertion compounds having an inverse spinel structure can unexpectedly exhibit substantial high voltage capacity that can be accessed at reasonable rate. These inverse spinel compounds may have similar stoichiometry to the aforementioned transition metal substituted LiMn.sub.2 O.sub.4 compounds, but the crystal structure differs significantly from that of spinel.
It appears that the electrochemical properties of most substituted LiMn.sub.2 O.sub.4 spinel compounds (including nickel and chromium substituted compounds) have not been determined above 4.5 V with respect to lithium despite the extensive research performed thereon to date. Thus, such compounds apparently have not been considered for use in lithium batteries operating in this high voltage range. Historical difficulties arising from a lack of stable high voltage electrolytes or previous negative capacity observations at lower voltages have tended to indicate that voltages above 4.5 V are not feasible.